The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising a marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,118,071; 4.043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate composition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,943,595, which is assigned to the same assignee as this application, discloses a method for explosively expanding formation containing oil shale to form an in situ oil shale retort. According to a method disclosed in that patent, an in situ retort is formed by excavating formation to form a columnar void bounded by unfragmented formation having a vertically extending free face, drilling blasting holes adjacent the columnar void and parallel to the free face, loading the blastholes with explosive, and detonating the explosive. This expands the formation adjacent the columnar void toward the free face in layers severed in a sequence progressing away from the free face so that fragmented formation particles occupy the columnar void and the space in the in situ retort site originally occupied by shale prior to such explosive expansion. The void fraction or void volume in the fragmented mass corresponds to the volume of the columnar void formed before explosive expansion. The void fraction in the resulting fragmented permeable mass is determined by the volume of formation removed from the retort site to form a void space toward which unfragmented formation remaining in the retort site is explosively expanded, inasmuch as such unfragmented formation is fragmented and expanded to fill such a void space. The original void volume is essentially distributed between the fragmented formation particles and the retort being formed.
U.S. Pat. No. 4,043,598 discloses a method for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to a method disclosed in that patent, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated, preferably in a single round, to explosively expand each unfragmented zone into the voids to form a fragmented mass. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. Pat. No. 4,192,554 issued to me on Mar. 11, 1980, describes a method for forming an in situ oil shale retort by expanding formation toward vertically spaced apart voids. U.S. Pat. No. 4,192,554 is incorporated herein by this reference.
It has been found that when forming in situ oil shale retorts, the fragmented permeable mass of formation particles can have a maldistribution of void fraction. For example, a fragmented permeable mass of formation particles can have vertically extending regions having a higher void fraction than regions which are adjacent laterally to such vertically extending regions. This maldistribution of void fraction can lead to higher gas flow in the region of higher void fraction than in the other regions.
When this occurs, a combustion zone formed in an upper portion of the fragmented permeable mass can advance more rapidly through the vertical region having a higher void fraction than through adjacent regions of lower void fraction. This can result in warping or skewing of the combustion zone as it advances through the retort.
It has been found that the yield of liquid and gaseous products from oil shale tends to be maximized when the combustion zone extends across the entire fragmented permeable mass and moves through the retort as a substantially planar wave. When the combustion zone is not planar, the yield of liquid and gaseous hydrocarbon products from the oil shale tends to be reduced. This reduction occurs firstly because the oil shale in some portions of the retort can be bypassed by the non-planar combustion and retorting zones and secondly because some of the shale oil produced by one portion of the combustion zone can be consumed by oxidation in another portion.
In upper regions of the fragmented mass, the locus of a combustion zone can be controlled by varying inlet gas composition and/or flow rate to various portions of the fragmented mass. Thus, for example, a secondary combustion zone can be sustained in a portion of the fragmented mass for controlling the rate of advancement of a primary combustion zone in adjacent regions of the fragmented mass. A secondary combustion zone is sustained by including a fuel in the retort inlet mixture. As used herein, the term "primary combustion zone" refers to a portion of the retort where the greater part of the oxygen in a retort inlet mixture that reacts with residual carbonaceous material in the retorted oil shale is consumed. As used herein, the term "secondary combustion zone" refers to that portion of the retort where fuel in the retort inlet mixture is consumed.
Control of the locus of the primary combustion zone by use of a secondary combustion zone is believed to be ineffective at distances greater than about 40 to about 60 feet below the top of the fragmented mass.
It is, therefore, desirable to provide a method for flattening a primary combustion zone advancing through an in situ retort when the combustion zone is a substantial distance below the top of the fragmented mass.